Vapor losses from crude and product storage tanks result in serious material and economic losses as well as air pollution. Evaporation from the liquid surface occurs during the storage of crudes and products in cone roof tanks. The maximum concentration of the evaporated hydrocarbons in the vapor space of the tank is determined by the vapor pressure of the liquid at the surface. The vapor pressure of the liquid is the saturation pressure exerted by vapors which are in equilibrium with the liquid. The maximum concentration or saturation concentration increases in direct proportion to the vapor pressure of the liquid at its free surface. Thus, the vapor losses should increase rapidly as the vapor pressure increases. These vapor losses include both "filling" and "breathing" losses. However, other forces in addition to vapor pressure are in action. Diffusion and convention of the hydrocarbon vapors from the surface throughout the vapor space is never complete. An essentially saturated vapor layer is in contact with the liquid surface. This saturated vapor layer slows the transfer of hydrocarbons to the vapor space above. Another mechanism, which often is the one controlling evaporation from crudes, is diffusion or transport of low molecular weight hydrocarbon compounds from the bulk of the liquid to the liquid-vapor interface. Evaporation lowers the concentration of these volatile hydrocarbons in the crude at the crude-vapor interface, thus lowering the vapor pressure and the saturation concentration. The transfer of additional high vapor pressure components from the body of the crude to the crude surface is generally slow relative to the vapor space.
It is known that evaporation losses may be reduced by floating a bulky material on the free surface of the liquid. Bulky low density materials are effective vapor barriers because (1) the portion of the material floating below the surface greatly impedes and thus decreases the transfer rate of the low molecular weight high vapor pressure components to the free surface of the stored petroleum where subsequent evolution can occur, and (2) the portion floating above the free surface greatly impedes the diffusion and convection of the evolved vapors from the free surface. Because of such actions, the evaporation losses from petroleum and products can be reduced substantially by floating low density particulate materials at the surface.
Analyses have shown that the composition of vapors evolved from medium gravity crudes are very similar. Data for several crudes show the vapors are similar to butane. About 80% volume or more of the hydrocarbons evolved are propane and heavier and have average molecular weights ranging from about 48 to 58. There appears to be no correlation between the composition of the vapors and either crude surface temperature, time of year, or crude type. Whereas the vapors lost from medium gravity crudes are slightly lower in molecular weight than butane (50 v 58), the vapors lost from gasoline are slightly higher than butane (65 v 58).
The use of bulky low-density materials as vapor barriers has been known for some time, and in the late 1950's the API Evaporation Loss Sub-committee III compiled many industry reports pertaining to the effectiveness of microballoons in reducing evaporation loss from field storage tanks. The microballoons were usually small phenolic balloons, less than one millimeter diameter, or urea balloons. In summary, it was found that for crude oils, one inch layers of microballoons reduced breathing losses 55% to 95% in stagnant tanks and evaporation losses in working tanks 50% to 85%. A reduction of 50% to 60% in evaporation loss for working tanks handling gasoline was reported when one- to two-inch layers of balloons were used. A major problem was loss of balloons due to water wetting and subsequent sinking, by becoming filled with fractions of the liquid on which they floated and subsequent sinking, and due to entrainment in the petroleum products taken from the tank. Another factor was cost effectiveness. These materials were expensive to install resulting in a payout time unfavorable to their use. Thus, microballoons were never used extensively throughout the industry.
The present invention in response to the above described need in the art overcomes the difficulties with microballoons and provides a successful solution to the problems of the art.